In monopulse tracking radars, the measurement of the angular deviation of a target from the antenna axis is based on the comparison of a sum signal .SIGMA. and a difference signal .DELTA. derived from echo signals received with two separate radiation diagrams. The angular-deviation calculation is carried out by determining for each pulse transmission the ratio (.DELTA./.SIGMA.) and averaging the resulting quotients over a given number of transmission recurrences. In operation, the signal is still mixed with the noise of the receiver and the calculation of the elementary angular deviation for each recurrence still suffers from errors, particularly in the case of low levels of the difference signal .DELTA., inasmuch as the calculation of the mean value of the elementary angular deviations does not allow the result of the final calculation to be corrected in a manner taking into account the nonlinear relationship between the ratio (.DELTA./.SIGMA.) (as a function of the deviation to be measured) and the signal-to-noise ratio.
In existing tracking radars, the digital processing of the angular-deviation signal entails problems linked with the choice of the number of definition bits necessary for this processing. In general terms, the sum signal .SIGMA. has, for example, a dynamic range of about 60 dB for the ratio of the maximum to the minimum amplitude of the detected signal. The search made preceding the tracking mode of such radars necessitates, particularly for the coding, the Doppler filtering and the contrasted detection of the signal, a digital coding range of 10 amplitude bits plus one sign bit. The least-significant bit is then of the order of the minimum signal level, i.e. the thermal-noise level of the radar. However, the level of the difference signal .DELTA. is substantially lower than the level of the sum signal .SIGMA., particularly when, during the tracking phase, the axis of the antenna is pointed onto the target. The angular-deviation signal representative of the offset of the target from the antenna axis is then represented by the amplitude ratio of the signals .DELTA. and .SIGMA.. An order of magnitude of the angular-deviation signal (.DELTA./.SIGMA.) is -20 dB for a signal received from a target in line with the axis of the radar antenna.
A known procedure for the retrieval of angular-deviation signals comprises coding the difference and sum video signals .DELTA..sub.i and .SIGMA..sub.i or the difference and sum video signals .DELTA..sub.j and .SIGMA..sub.j respectively obtained by amplitude demodulation of in-phase and quadrature signals, followed by the processing of these components in a digital Doppler filter before computing the ratio (.DELTA..sub.i /.SIGMA..sub.i) or (.DELTA..sub.j /.SIGMA..sub.j) representing the angular-deviation signal for each occurrence. However, for signals .SIGMA. close to the detection limit, i.e. for a signal-to-noise ratio close to 0 dB, the resolution threshold of the coding and the processing necessary for searching purposes is of the order of the signal level corresponding to the least-significant bit equal to the effective noise value. The resolution threshold needed for the coding of the difference signal .DELTA. is then 15 dB lower, the coding and processing of the angular deviations therefore requiring two to three supplementary information bits.
Thus, it is not possible to use the processing circuits of the signal of the search mode for the tracking mode, more particularly the analog-digital converter and the digital Doppler filter, because the signal for the tracking mode is at a level substantially less than the level corresponding to the least-significant bit of the search coder, i.e. at a level greatly below the resolution threshold of the processing circuits of the search mode.